Add getfromjson code for Lambda-backed custom resource consumers.

[mrinaudo-aws]

Issue #, if available:

Description of changes:
Add getfromjson code for Lambda-backed custom resource consumers.

Unit tests excerpts

% pytest --cov
[...]
collected 18 items                                                                                                                  

src/tests/test_getfromjson.py ..................                                                                              [100%]

[...]
Name                 Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------------
src/getfromjson.py      63      0     18      0   100%
----------------------------------------------------------------
TOTAL                   63      0     18      0   100%

Required test coverage of 100.0% reached. Total coverage: 100.00%

[...] 18 passed in 0.19s [...]

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